The invention relates to a device and a method for measuring the amounts of milk in particular during the milking process. The invention is generally suitable for determining the quantity of milk yield from cows, ewes, goats, buffaloes, llamas, camels, dromedaries, or of other lactating mammals and will be described below with reference to the milking of e.g. cows.
In addition the invention may be used in other areas of application where the measuring of quantities or mass flows of foamy or foam-forming liquids is concerned such as measuring the mass flow of beers, soft drinks, fruit juices, or other similar food products, as well as foaming or foamy technical fluids, such as electroplating solutions.
Information about the current milk flow is important for controlling the milking process so as to adjust parameters during milking e.g. for specifying the transition from the stimulating phase to the main milking phase, or the removal time. Although a high accuracy level is usually not required, it is desirable.
Milk yield measuring is significant for drawing conclusions on the performance of the individual cows. It is useful to have accuracies meet the requirements of ICAR since this would eliminate the need of regular separate milk yield measuring. Although the requirements for an ICAR (International Committee for Animal Recording) approval depend on the animal variety and other parameters, they are generally high. The allowable error for cows and for milk yields larger than 10 kg, is 2 percent maximum at a maximum standard deviation of 2.5%. As a rule, however, a general assessment of performance or controlling the milking process does not require this level of accuracy.
One advantage of measuring the amount of milk or the mass flow of milk is that in individual cases, the shape of the milk curve during milking or the total milk yield will allow to draw conclusions on the state of health of the animal.
One problem encountered in milk flow measuring is that milk is a heavily foaming fluid. Foam formation during milking is further intensified by the currently employed milking techniques since as a rule, air is periodically or continually allowed to enter into the milk collection piece and/or the teat cups during milking to discharge the milk. Although the volume of air intake per unit time may vary, it will as a rule be approximately 8 liters of air/minute or even higher. Assuming a maximum milk flow e.g. for cows of approximately 10 or 12 liters of milk per minute in the main milking phase, the air volume to be discharged will roughly be at least approximately 25%, in particular at least 40% or even 50% of the volume flow of milk yield and air intake. And this quite considerable portion is already present during the maximum milk flow phase. Near the end of milking the proportion of air to be discharged will rise even higher due to the decreasing milk flow. Added to this there is the proportion of air entering at the teat cup due to less than tight sealing between the liner and the teat. This proportion can also be roughly estimated at e.g. 10 liters of air per minute. The considerable proportion of air in the air-fluid mixtures to be discharged may thus cause not only individual foam bubbles but substantial formation of foam which considerably impairs in-flow milk yield measurement.
Since the proportion of foam does not readily permit conclusions on mass from the volume, the accuracy of milk yield measuring methods through volumetric methods has its limits. Both the proportion of air in the fluid and the bubble size in the foam are not always even but they depend on a plurality of factors. These factors include, the milk temperature, the milk flow rate, the position and layout of the milk hoses, the type of milking unit, the type of teat liner, the milk hose diameter, the type of milking installation, the vacuum level and the pulse rate during milking, air leakages or air infiltration, the current state of health of the udder, individual differences between cows e.g. due to the lactation stage or the race of the cow, and due to differences in kind and quantity of feeding, etc.
Another problem in measuring the milk yield flow is caused by the periodic milk flow. Unlike measuring volumetric flow in many other applications, milk is drawn periodically. The pulse space and thus also the teat space in the teat cup is subjected to a periodic vacuum such that milk will flow out of the teat approximately at the pulsation rate. The pulse rate typically lies between approximately 30 and 90 at e.g. 60 cycles per minute. Given four teats and identical rates with all of the teats, there will be a milk flow having approximately 60 milk flow pulses per minute. Where the udder halves or the four teats e.g. of a cow are selected variably, the high frequency proportion of the milk flow may increase to reach approximately 240 strokes per minute at a pulse rate of 60. Milk is often conveyed through the milk hoses in clusters such that short phases at maximum milk flow alternate with short phases at minimum milk flow. Determining the actual milk flow is difficult under these conditions.
Due to these influences, measuring a milk flow is found to be difficult since the nature and composition of the foam phase on the one hand and on the other hand also the composition and quality of the liquid phase within one milking process and between milking processes are subject to fluctuations. For example the electrical conductivity of the fluid and the proportion of the foam phase may vary continually since e.g. the fat content may change during milking which will cause fluctuations in terms of the electrical conductivity and the optical properties of the milk. Measuring methods based on measuring these parameters may thus be subjected to not inconsiderable inaccuracies.
DE 30 20 161 C2 discloses a device for milk yield measuring where milk is periodically dammed in a chamber until it is filled to then discharge the content of the chamber. By way of the time required for filling and the volume contained in the chamber the total amount of milk can be summed up and the current milk flow assessed. The achievable accuracy of such measuring devices is high. Owing to the principle, however, periodically operating measuring methods do not allow precise determination of the current milk flow in particular with slight milk flows. Exact information on the current milk flow is, however, helpful at the start of milking and in particular at the end of milking for adjusting the process parameters and for specifying the best time of removal. In-flow measuring may provide better control of the removal process since continuous measuring enables early recognition of the best time for removal.
EP 0 536 080 A2 discloses in-flow milk yield measuring wherein the milk is conveyed through flow channels, and transmission of an infrared light beam through the milk is measured and analyzed. The temporarily dampened or dimmed infrared light beam through the channel as milk is flowing through allows to draw conclusions on the momentary mass flow of milk through said channel. One drawback of optical measuring is for example that small and large foam bubbles may scatter the light beam employed for measuring such that in the presence of a foam portion, not enough light can be measured in transmission or reflection measuring so as to result in measuring errors.
DE 37 37 607 A1 discloses another method and a device for in-flow milk yield measuring. A plurality of electrodes positioned one above the other is provided to firstly determine the electrical impedance or electrical conductivity of the liquid and air mixture on the respective levels by means of the electrodes. In the bottom region, a reference conductance of the liquid currently passing is measured. On the basis of each height value the stepped level profile of the specific impedance is calculated by means of the reference conductance. The flow velocity of the draining off liquid is known for known impedance profiles from calibration measuring such that the stepped level profile permits conclusions on the flowing mass of the milk.
A device operating on this principle has become known wherein a plurality of approximately 60 electrodes is positioned one above the other. To cover a wide range of heights and thus of measuring, the electrodes are spaced apart from one another in the vertical direction by a specific electrode-free distance. The actual condition at measuring cannot be detected in said electrode-free area. Given a distance of e.g. 1.5 mm, this may mean a measuring error of nearly 1.5 mm, which with slight milk flows at low fill heights of a few millimeters may result in correspondingly large measuring errors.
Due to this principle, this known device is very complex in terms of mechanics and electronics since a large number of electrodes must be installed and individually selected electrically. It is another disadvantage that despite the high number of employed electrodes the milk flow is only evaluated in steps, thus causing inaccuracies.
It is therefore still a major challenge to integrate a sensor in the milk flow offering satisfactory measuring accuracy at a reasonable price for farmers.